The present invention relates to spectrum analysis, and more particularly to a method of displaying a marker having variable time and frequency scales on a graph of a signal under test, and to displaying sub-graphs relating to data designated by the marker.
In wireless communication using digital modulation, such as that represented by a cell phone or wireless LAN, wireless signals are complicated in that they limited to a finite frequency resource and therefore use either multiple-carriers or time division multiplexing with a burst signal. FIG. 1 shows an example of a display presented by a conventional spectrum analyzer used for analyzing these complex signals. The display is divided into three view areas—A, B and C. View area A shows a three-dimensional spectrogram display where the X or horizontal axis indicates frequency, the Y or vertical axis indicates time and a variation of color or brightness indicates magnitude of power. The spectrogram provides an overview of time, frequency and power of a signal under test. View area A has a pair of time cursors 12 and 14, and view areas B and C show the analytical result of processing the data within the area delimited by the cursors, i.e., view areas B and C are sub-graphs for view area A. Specifically, view area B presents a display of power vs. frequency and view area C presents an analytical display in a modulation domain, such as a constellation of symbols, EVM (Error Vector Magnitude), etc. The display of FIG. 1 is suitable for analyzing a time variable signal like a burst signal, but not for analyzing each carrier in a multiple-carrier signal because there are no means to delimit an area, or band, in the frequency axis direction.
FIG. 2 shows an example of displays provided by another conventional spectrum analyzer that also divides the display into three view areas—A, B and C. View area A presents a spectrogram that has a pair of frequency cursors 16 and 18 that designate a band of frequencies under analysis, with the results being displayed in view areas B and C. The display of FIG. 2 provides a zoom function, i.e., a graphical extended analysis on the designated frequency band along the frequency axis by reprocessing the time domain data stored in a memory for that frequency band using a time-to-frequency conversion algorithm, such as a Fast Fourier transform (FFT). Specifically, view area B displays an extended power vs. frequency graph of the designated frequency band in the signal under test. View area C displays a constellation of the symbols within the designated frequency band in the signal under test. This display is suitable for analyzing a signal transmitting information in a frequency divisional scheme, such as multi-carriers, but it does not consider separation on the time axis.
FIG. 3 shows an example of displays by another conventional spectrum analyzer, the technology of which is disclosed in “A 3D Spectrum Analyzer Prototype for 3G Mobile Telecommunication Systems”, IMTC2004-Instrumentation and Measurement Technology Conference, Como, Italy, 18-20 May 2004. View area A is a spectrogram derived from a kind of fast Fourier transform, or the Wigner-Ville Distribution, changing a window function according to the signal under test. This display provides three-dimensional data on cross-sections of frequency and time axes at the same time. View area B of FIG. 3 shows a power vs. frequency display at the time of a dotted cursor 20, and view area D shows a power vs. time graph at the frequency of a dotted cursor 22. However, FIG. 3 does not present a display that designates time and frequency intervals that have desired widths. Further the display of FIG. 3 cannot provide a display in the modulation domain.
A signal analyzer suitable for providing the displays shown in FIGS. 1 and 2 is disclosed in U.S. Pat. No. 6,484,111 (Akira Nara), for example. The signal analyzer produces frequency domain data by an FFT processing circuit from corresponding time domain data. The signal analyzer has means to feed back the time domain data once stored in a memory to the FFT processing circuit so that it recreates a desired range of frequency domain data in the signal under test that a user want to analyze.
The above conventional display methods do not provide an effective analysis for a complex signal multiplexing frequency and time dimensions. The method of FIG. 1 does not analyze a plurality of carriers on the frequency axis by separating them. In other words, the method of FIG. 1 divides time into desired intervals to obtain proper time resolution, but not proper frequency resolution in exchange for the time resolution. The method of FIG. 2 extracts data on the frequency axis, but not on the time axis, i.e., too much time data decreases time resolution and it is difficult to set a proper time resolution according to the measurement. The method of FIG. 3 displays cross-sections at a desired time and frequency, but is not suitable for analyzing a signal having some bandwidth, and does not handle modulation domain analysis.
Therefore, what is desired is a method of extracting a desired portion of a signal under test effectively from the viewpoints of both the time and frequency domains to analyze and display it.